Process and apparatus for filtering effluent produced
from aluminum reduction cells

ABSTRACT

THIS INVENTION COMPRISES AN APPARATUS ASSOCIATED WITH (AN) A METALLURGICAL FURNACE SUCH AS A ALUMINUM REDUCTION CELL AND A PROCESS FOR DIRECTING THE HOT GASES, VAPORS AND DUST ISSUING FROM THE (CELL) FURNACE AND PASSING THEM THROUGH A WALL OF PULVERULENT (ALUMINA) ABSORBENT SUPPORTED BETWEEN UPRIGHT METAL WALLS HAVING A MULIPLICITY OF OPENINGS. (THE) IN AN ALUMINUM REDUCTION CELL, THE WALL OF ALUMINA WHICH MAY CONTAIN SOME ADMIXED ACTIVATED ALUMINA, PROVIDES A PHYSICOPHYSICAL FIXTURE WHICH ENTRAINS THE DUST, ABSORBS THE FLUORIDE VAPORS AND GASEOUS COMPOUNDS, ABSORBS THE HEAT AND RETURNS THE ABSORBED CHEMICALS, DUST AND HEAT TO THE CELL. THE WALL OF ALUMINA IS PREFERABLY SUPPLIED FROM AN UPPER FEED HOPPER AND AS THE WALL OF ALUMINA MOVES DOWNWARDLY IT ENGAGES MEANS WHICH CONTROLS, FEEDING THE WALL OF ALUMINA INTO THE CELL WHICH ALSO MAINTAINS THE WALL OF ALUMINA. THE METAL WALLS PREFERABLY HAVE MANY SPACED UPWARDLY DISPOSED BAFFLES WHICH PREVENT SPILLAGE OF THE ALUMINA AND INTERVENING SPACES THROUGH WHICH THE GASES, VAPORS AND DUST PANS INTO THE ALUMINA.

Original Filed F013. 6, 1967 May 30, 1972 A. F. JOHNSON Re- 27,383

PROCESS AND APPRTUS FOR FILTERING EFFLUENT PRODUCED FROM LUHINUM REDUCTION CELLS 2 Sheets-Sheet l lNVENTOR Arthur F. Johnson HWI/143m, 21mm v fag/@az Ja/u..

ATTORNEYS .May 30, 1972 A F, JOHNSON Re. 27,383

PROCESS AND APPARATUS FOR FILTERING EFFLUENT PRODUCED FROM ALUMINUM REDUCTION CELLS Original Filed Feb. 6, 1967 2 Sheets-Sheet 2 INVENTOR Arthur F. Johnson BY 1MM' ML ATTORNEYS [United States Patent Office Re. 27,383 Reissued May 30, 1972 Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue speciication; matter printed in italics indicates the additions made by reissue.

ABSTRACT 0F THE DISCLOSURE This invention comprises an apparatus associated with [an] a metallurgical furnace such au a aluminum reduction cell and a process for directing the hot gases, vapors and dust issuing from the [cell] furnace and passing them through a wall of pulverulent [alumina] absorbent supported between upright metal walls having a multiplicity of openings. [The] In an aluminum reduction cell, the wall of alumina which may contain some admixed activated alumina, provides a physicophysical tixture which entrains the dust, absorbs the fluoride vapors and gaseous compounds, absorbs the heat and returns the absorbed chemicals, dust and heat to the cell. The Wall of alumina is preferably supplied from an upper feed hopper and as the Wall of alumina moves downwardly it engages means which controls feeding the wall of alumina into the vcell which also maintains the wall of alumina. The metal walls preferably have many spaced upwardly disposed baffles which prevent spillage of the alumina and intervening spaces through which the gases, vapors and dust pass into the alumina.

The apparatus of the invention as used with an aluminum reduction cell, for example, comprises a hoodlike structure over the cell for receiving and coniining the hot eiuent Vincluding gases, vapors and dust issuing from the cell and upright baffled walls for confining the wall of alumina, a receptacle for feeding the alumina into the space between the walls, and means for forcing or drawing the hot eflluent through the wall of alumina. The Wall of alumina is preferably located directly over the cell so that it can be fed into the fusion in the cell. The baffled walls may be formed of metal screens of a type having many closely spaced upwardly facing batlles and intervening slots or they may be a fabricated stationary or adjustable part of the Walls. The metal walls may have' open slots comprising 60 to 80% of the wall area. An important characteristic of the bai-lies is that they slope upwardly and outwardly at such an angle that they approximate the angle of repose of the alumina and the alumina is, accordingly, prevented from owing out but the hot effluent can flow in through the spaces. The angle of repose of alumina is about 30 and the baffles should be set at such an angle and be individually dimensioned and spaced apartso that at the speed of the hot effluent the alumina does not become iluidized and overow the baffles. One type of baliled screen which may be used is formed of expanded sheet steel and is illustrated in Patent 2,366,224, issued to H. Warp.

The filter apparatus of the invention is in operative connection with a metallurgical furnace, as for example, a cell for the electrolytic reduction of aluminum in a fused electrolyte and particularly with the hood enclosing the upper part of the cell for collecting the hot ellluent from the cell and the bin for supplying the cell with alumina.

The filtering apparatus may be located remote from the cell and connected therewith by a duct but preferably is,y mounted above the hood and supported by the hood, comprising a plurality of pairs of upright metal baffled walls each of which contines a wall of granular alumina, means for passing the hot effluent from the hood through the alumina walls whereby the hot eluent is absorbed by the alumina walls, means for supplying alumina from the bin to the baffled walls to maintain them iilled with alumina and means, preferably a supporting bin below the lters with feed regulating valves, for feeding the alumina of the walls together with their absorbed hot eilluentinto the cell.

'Some of the advantages of the invention are:

(1) The temperature of the process of filtration of hot eliluent is limited only by the corrosion resistance and physical strength of metal used in the baflied walls.

(2) The baied walls of the filters being compact and resistant to heat may be a part of the ore bin of each reduction cell thereby eliminating the bag house and associated equipment for its operation, and the difliculty of returning the materials recovered for return to the reduction cells.

(3) Because of the above advantages it is possible to reduce greatly the amount of cooling air allowed to enter the hood covering the cell and required to cool and dilute the hot eiiluent at 800 to 1000 C. before it may be ltered. The invention makes unnecessary cyclone dust collectors, bag houses, electrostatic precipitators or spray towers as conventionally used and requires practically no additional moving parts.

The invention will be better understood after considering the following description and drawings in which FIG. 1 is a vertical section through apparatus of the invention as used with an aluminum reduction cell;

FIG. 2 is a cross section at 2-2 of lFIG. 1;

FIG. 3 is an enlarged fragmentary vertical sectional view of a pair of baffled walls taken at 3-3 of FIGS. 1 and 2, and

FIG. 4 is a vertical sectional View of modied baled Walls with adjustable baffles.

FIGS. 1 and 2 illustrate an assembly of apparatus of the invention located in the superstructure above an aluminum reduction cell 1 comprising the steel shell 2 and steel deck plate 3 enclosing the carbonaceous or other refractory lining which contains the molten fusion being electrolyzed which fusion freezes on its upper surface to a crust 4 on which the freshly fed alumina 5 rests and in which the anodes 6 are suspended by anode rods which conduct current into the fusion to electrolyze the alumina to molten aluminum. The filter assembly F of the invention includes several pairs of vertical walls 8 as best shown in FIG. 3 made of batlled steel plates which pairs each confine therebetween a wall or column of alumina 9. The baffled walls 8 are mounted between upright steel Walls 12 which extend downwardly and are supported by the bin 13 which holds the supply of alumina for the cell. An upwardly tapered eiiluent collecting hood 14' is mounted on the deck plate 3, and the upper ends are connected to the side walls 15 of the alumina bin. The sloping Walls of the hood are preferably formed of inner continuous plates 16 which are in connection with the deck plate 3 and with the walls 15. This connection with the deck plate need not be tight as some infiltration of air into the hood which mixes with the hot effluent can be beneficial. Exterior plates 17 are secured in spaced relation to the plates 16 to form channels 18 through which cooling air can circulate upwardly from the opening below 19 to the opening 20 above. As best shown in FIG. 1, hoppers 22 are mounted below the filter 8 and are connected to the walls 12 to receive the alumina 9 of the filters which passes downwardly. Feed valves 23 in the rm of tip troughs are mounted in the hoppers 22 which UVor may be changed to better clean all the alumina olf the Y Y bafiies when the yfilter is emptied.

tlves are operated from time-to-time to feed the 'alumina' tto the cell.

Asshown in FIG. 3 the adjacent pairs of walls 8 are osed at their ends by upright steel plates or walls 12', le opposite'ends having openings O- and adjacent pairs Ewalls 8 are closed by plates 12', the opposite ends havlg openings O'.V The walls 12 are spaced from the walls to provide spaces S for the hot eliiuent to enter the peningsv O and pass through the filters. The walls 12 )rm the sides of an upright duct D with which the penings'O connect and are outlets for the residual ises of the'filtered hot effluent. One end of the duct D )nn'ects to a longitudinal header (not shown) serving averalcells, which connects to a stack 25 in which a fan iduces adraft to suck the hot efiiuentfrom the hood trough the filters and discharge the inertl gases to the mosphere.

The embodiment of the invention illustrated in FIGS. 2 and 3 4has baffled walls in the form of steel screens referably made according to said Patent 2,366,224 As town in the enlarged scale in FIG. 3, the upright walls are identical and consist of louvers or baflies 26 and lterposed openings 27. The baies 26 slope upwardly 1d outwardly at angles 0 which are at least equal to the 1gle of repose of granular alumina. The angle of rease for such alumina is about 30. Since the alumina tay containadmixed fine particles of activated alumina, ie angle may be enlarged 5 to 15% to insure the retenon of the confined wall 9 of alumina. The spaces beveen the walls 8 may vary from less than one inch to :veral inches depending on the operating conditions. he shortest distance, a, of gas flow through the alumina 1d the longest straight distance, b, should be a multiple E several times the bafiie Widths, c, so that the gas being ltered will not channel but move relatively uniformly trough all the granular alumina held between the bafile :reens. Since such baiiie screens are economically manutctured by slitting a sheet of metal a distance, d, and ending it outward, c equals d approximately. The arca E openings in the screen through which gas may flow reduced by a greater thickness of metal, e, or a greater igle from the horizontal 0. The metal used to manuictiure the baffles may be steel, anodized aluminum or :her metals but steel is usually the most economical since can be made relatively thin such as 0.01 inch and the istance c and d made 0.04 to 0.10 inch or more. The /erage thickness of the alumina wall 1/2 (a-t-b) may be to 0.50 or more but 0.25 inch will usually insure that 3 part of the screen is blocked by foreign matter. This important since a blow-through must not be permitted continue long in any filter Wall. The porosity of cell ade alumina is about 50% and the resistance to flow E the gas through the above dimensioned example of iter wall ata velocity of one foot or less a minute will L the -most `only amount to a few inches of water preslre head or even less than l inch depending on the finess of the alumina as Well as other enumerated variarles.

'In conventional Ifiltering in bag houses it is the acxmulation of microscopic fume particles in the fabric are spaces that inhibit gas flow and cause back preslre. An important feature of this invention `is that the ,umina used in the cell operation is used as a filter which :turns the absorbed chemicals, dust and heat to the lsion.

FIG. 4 illustrates a modification of filter walls 30 in hich the bafiies 311 are mounted on pivots 32 connected the walls 12 and 12 and the outer ends are pivotally )nnected to upright rods 33. These rods may be moved p or down as in a jalousie window to move all the baiiies t unison to the desired slope. In this modification the ope of the filter bafiies may be changed depending on e character ofthe alumina as it affects its angle of repose In one method of practicing this invention about 100 parts of conventional granular cell grade alumina containing only 3% to 20% of minus 325 mesh particles and calcined to about 0.5% loss on ignition is mixed with 10 to 20 parts of activated alumina which may be about the same chemical grade of alumina but substantially all of a size less than 325 mesh and comparatively undercalcinecl so that the activated alumina has from 1.5% to 10%, preferably 4% to 8%, loss on ignition. As disclosed in Canadian Patent 613,352, issued Jan. 24, 1961, the activated alumina is an efficient absorber of fluoride gases. By mixing the activated alumina with conventional cell grade alumina dusting of the fine activated alumina is avoided.

In another methodof practicing this invention only conventional cell grade alumina is utilized but this is slightly undercalcined to contain 1.0% or more loss on ignition. The atmospheric vair infiltrating around the hood of the cell is then held=down so thatV the `temperature of the hot effluent to be filtered is 350 C. or more whereupon the undercalcined alumina is converted to aluminum fluoride by the fluoride gases to the extent these gases are present. l

In iFIGS. l, 2 and 3 it is apparent that not all the bafi'le screen face is available for filter area since the slotted portions must necessarily be supported by unc-ut metal sheet. Itis, however, easily possible for the baffle openings lto constitute to 80% of the screen sheet area.

It is an important feature of this invention that the assembly of spaced filters are automatically sealed above by the alumina in the ore bin which rests on the walls of alumina 9, and below by the alumina which fills the hopper-and on which the walls of alumina 9 rest. Thus the alumina used for reduction is forced to pass. between the spaced batiie walls through which the finoride containing hot and dusty gases are drawn so the heat and fluorides and d-ust are caught and enter the fusion with the alumina.

Although a utilization of this invention for the recovery of iiuorides from aluminum reduction cells is described above, it is likewise possible to recover iluorides from effluent gases of other metallurgical operations such as fertilizer plants treating calcium fluoapatite. In this case limestone particles may be used between the baflied screens instead of aluminav and biproduct calcium fluoride manufactured.

In any case the filter assembly may be located remote from the cells or furnaces from which luorides are evolved with interconnecting ducts being used to carry the effluent gases, fume and dust. In some cases this may be advantageous Where conventional cells and ductwork are already in existence and it is desired to replace conventional gas washing, electrostatic treatment or bag house filters with the filtering process of this invention without greatly altering the cells. In this case the process of this invention has great utility in permitting smaller volumes of gas drawn from each cell or furnace since the fusion of the aluminum reduction cell from which they effluent was collected. v

2. The process of claim 1 in which activated alumina is mixed lwith cell grade alumina to assist the recovery of `fluorides.

3. The process of claim 1 in which only cell grade alumina slightly undercalcined but less than 3% loss on ignition is utilized in the filter, and the hot effluent filtered has a temperature of at least 350 C.

4. A physico-chemical process for recovering the hot effluent including fluoride gases,` vapors, dust and heat from electrolyzing alumina in a fusion containing at least some fluorides which comprises diluting the hot effluent with some atmospheric air, passing the diluted hot effluent through a filter wall of granular alumina held between conlining upright metal walls, thewall of alumina effecting a physico-chemical absorption of the iluorides, dust and heat, feeding the alumina by gravity from a supply above into the upright metal walls to maintain the walls of alumina, and passing thel alumina with its contained liuorides, dust and heat by gravity into the fusion in the reduction cell.

5. The combination with a cell for the electrolytic reduction of aluminum in a fused electrolyte which comprises a hood enclosing the upper part of the cell for collecting the effluent from the cell and any air infiltrated therein, a filtering apparatus supported above the hood having a plurality of pairs of upright metal walls for combining a body of granular alumina, means for passing the efiiuent from the hood through the alumina body whereby the eflluent and heat are absorbed by the alumina body, means for supplying alumina to the filtering apparatus to maintain it filled with alumina and means for feeding the alumina from said body together with the absorbed efiiuent into the cell.

6. The combination of claim 5 which comprises spaced upright bafiied walls with openings therein for confining the alumina body, said baies sloping upwardly at an angle at least equal to the angle of repose of the alumina and be so spaced that the alumina does not flow out through the openings.

7. The combination of claim 5 which comprises means associated with the hood to cool the electrolyte crust in the cell, a means for increasing the transmission of heat from the crust to the inside of the hood and increasing the removal of heat from the hood to outside to reduce the amount of cooling air.

8. The combination of claim 5 in which the filtering apparatus is located at a distance from the cell and is connected by duct means for receiving the efiiuent and passing the alumina into the cell.

9. Apparatus for the electrolytic reduction of aluminum from fluoride fusions which comprises a cell in which the fusion is reduced, a hood attached to the cell and extending upwardly therefrom for the collection of hot effluent from the reduction, a structure above the hood having Supported therein a plurality of upright filters, each filter consisting of two metal walls providing a space which is adapted to be filled with pulverulent alumina, each wall having a multiplicity of openings and means for confining the alumina, a receptacle above the filters for holding a supply of alumina which feeds into and maintains the filters filled with alumina, means for drawing the hot efiiuent from the reduction operation of the cell through the filters to absorb hot efiiuent and discharge the unabsorbed gases to the atmosphere, said filters being located above the cell so that the alumina in the filters can move downward by gravity carrying with it the absorbed hot effluent, and means to regulate the feeding of alumina from the filters into the cell so that they are maintained filled with alumina.

10. Apparatus as defined in claim 9 in which the metal walls have a multiplicity of outwardly and upwardly sloping baffles extending horizontally and openings between the adjacent baffles, the slope of the bafes being at least equal to the angle of repose of the alumina and their dimensions such that the alumina does not spill out of the filters.

11. Apparatus as defined in claim 9 in which the means to regulate the feeding of alumina into the cell is a holding bin for containing the alumina which has moved downward from the filters and for holding the alumina in the filters, and a feed means in the bin for periodically discharging the alumina from the bin onto the fusion crust in the cell.

12. A physico-chemical process for filtering and recow ing the eluent from a metallurgical furnace including vapors, gases, dust and heat which comprises collecting the effluent, passing the eluent through an upright confined wall of granular absorbent, and moving the wall of absorbent with the contained effluent into the fusion of the metallurgical furnace from which the efjiuent was collected.

`13. A physico-chemical process for recovering the hot effluent inclding gases, vapors, dust and heat from a fusion in a metallurgical furnace which comprises diluting the hot effluent with some atmospheric air, passing the diluted ho-t effluent through a filter wall of granular absorbent held between confining upright metal walls, the wall of absorbent effecting a physico-chemical absorption of the gases, vapors, dust und heat, feeding and absorbent by gravity from a supply above into the upright metal walls to maintain the walls of absorbent, and passing the absorbent with its contained gases, vapors, dust and heat by gravity into the fusion in the metallurgical furnace.

14. The combination with a metallurgical furnace which comprises a hood enclosing the upper port of the furnace for collecting the eluent from the furnace and any air infiltrated therein, a filtering apparatus supported above the hood having a plurality of pairs of upright metal walls for containing a body of granular absorbent, means for passing the effluent from the hood through the absorbent body whereby the effluent and heat are absorbed by the absorbent body, means for supplying absorbent to the filtering apparatus to maintain it filled with absorbent and means for feeding the absorbent from said body t0- gether with the absorbed effluent into the furnace.

15. The combination of claim 14 which comprises spaced upright baffled walls with openings thereinI for confining the absorbent body, said baffles sloping upwardly at un angle at least equal t0 the angle of repose of the absorbent and being so spaced that the absorbent does not flow out through the openings.

16. The combination with a metallurgical furnace in which a fusion is reduced, a hood attached to the furnace and extending upwardly therefrom for the collection of hot eluent from the reduction operation of the furnace, a structure above the hood having supported therein a plurality of upright lters, each filter consisting of two metal walls providing a space which is adapted to be filled with pulverulent absorbent, each wall having a multiplicity of openings and means for confining the absorbent, a receptacle above the filters for holding a supply of absorbent which feeds into and maintains the filters filled with absorbent, means for drawing the hot effluent from the reduction operation of the furnace through the filters to absorb hot effluent and discharge the unabsorbed gases to the atmosphere, said filters being located above the furnace so that the absorbent in the filters can move downward by gravity carrying with it the absorbed hot effluent, and means to' regulate the feeding of absorbent from the filters into the furnace so that they are maintained filled with absorbent.

17. A physical-chemical process for filtering and recoveling the effluent from aluminum reduction cell means including fluoride vapors and gases und' dust and for removing heat from said eluent which comprises collecting the eluent; passing the effluent through an amount of alumina sufficient for the operation of the cell means to entrain said dust and absorb said fluoride vapors und gases therein and t0 remove heat therefrom, said' alumina including a substantially stationary body through which the eluent is passed; and moving the alumina with the contained ejluent into the fusion of the aluminum reduction cell means from which the effluent was collected.

18. The process of claim 17 in which the'eluent is 'iluted with atmospheric air before being passed through aid alumina. t

19. The process of claim 18 wherein said body of :lumna is dimensoned relative to the velocity of the 5 References Cited A The following references, cited by the Examiner, are

If record in the patented le of this patent Or the 'original 1atent.

' 8 'f'UNITED STATES PATENTS "2,564,837 `s/1951 Ferrand 204-67 X 3,090,744 5/1963 Muller et a1. 204-243 3,311,466 "3/19671 cprl'ock l 75-26 l FOREIGN .PATENTS 592,973 2/1960 canada.

' 613,352 1/1961 canada.

10 JOHN H. MACK, VPrimary Examiner 1 1D, RQKVALENTINE,Assistant Examiner 1 U.s. c1. XR. 75425, 68;' 204-243 R, 245, 247 

